1. Field of the Invention
The present invention relates to an encoder.
2. Description of Related Art
Currently, as a type of an apparatus for measuring a displacement, an optical linear encoder is known. The optical linear encoder includes a scale and a detection head that moves along the scale. The scale is provided with, for example, an absolute pattern for detecting a reference position and an incremental pattern for detecting a relative displacement between the scale and the detection head. The optical linear encoder determines the reference position by using a reference signal which is a result of the detection of the absolute pattern on the scale. Further, a positional relation between the scale and the detection head can be detected by taking the above-described displacement into account while using the reference position as the reference.
An example of an optical linear encoder is an encoder that detects a scale grating having continuous periodicity by using a lens array optical system (Published Japanese Translation of PCT International Publication for Patent Application, No. 2005-522682). In this encoder, the pitch of the lens array is determined so that phases of formed images are synchronized or a specific phase shift occurs on the boundary of the range where the optical system projects the image by each individual lens constituting the lens array. In this way, a collection of local images formed by the lens array can be detected by using one light-receiving element array.
Further, an encoder that forms an image of a grating pattern by using a telecentric optical system has also been proposed (Japanese Unexamined Patent Application Publication No. 2006-284563).